Bad addresses might deserve to be atop the list.  Perhaps somebody gave you a fake email address; perhaps a typo or inactive account just goofed things up.  Regardless, Amit Agarwal warns, "[Y]our account will get disabled if the email message contains a large number of non-existent or broken addresses (<25 ?) that bounce back on failed delivery."

Now, as you probably noticed, that bit of advice mentions 25 addresses for one email.  It’s an easily foreseeable scenario.  But we’re heading into less likely territory.

Agarwal also writes, "If you access Gmail via POP or IMAP clients (like Microsoft Outlook), you can send an email message to a maximum of 100 people at a time.  Cross the limit and your account will be disabled . . . "  And, "If you access Gmail from the browser, you may not address an email message to more than 500 people at a time."

So extremely heavy users may want to take care.  Here’s one more piece of advice for the average person, though: be careful about using a Gmail account as a spam dump, because it will get completely deactivated if you don’t visit at least once every nine months.